Transducers of acoustic energy to and from a water medium are many and varied in design. Operational considerations such as a desired frequency response or pressure insensitivity, for example, have led to filling a transducer's housing with a liquid.
Obviously, not all liquids are satisfactory, particularly when they must couple the transducer's active element to its acoustic window. The liquid should have the right acoustic properties so that there is no convergence or divergence of the acoustic signal. The chemical composition of the fluid should have certain stabilities and compatibilities with other parts of the transducer. Handling the liquid should pose no hazards when the transducer is periodically repaired or serviced. For these and several other reasons, castor oil has been used for a number of years as the acoustic link in one type of transducers. However, castor oil tends to be too viscous at low temperatures. Even at room temperature castor oil is too viscous to fill all the voids and small cavities in a transducer. Another type transducer has an orifice between the driving element and the acoustic window and cannot use castor oil at all when it is operated in cold water. Another somewhat acceptable liquid that has been used in acoustic transducers is phenol polysiloxane. The acoustic and physical properties of this liquid are consistent with sound transducer design. However, it reacts with the butyl and buna rubber which is usually used as transducer sealing elements and, in addition, it causes skin and eye irritations. These two liquids are discussed only for comparison purposes to demonstrate that, while they come to standards in some aspects, they are deficient in other respects. For these reasons and several others not elaborated on they are not generally regarded as the best available. A variety of other liquids have been used in other liquid filled transducers, yet all have some shortcomings in the trade-off of balancing the required acoustic, chemical, physical, and environmental properties. A continuing need exists in the state of the art for a liquid which satisfactorily couples a transducer's driving element to its acoustic window.